1. Field of the Invention
The present invention relates to a transmission/reception optical module having an optical transmission subassembly, an optical reception subassembly, and a circuit board for controlling the optical transmission subassembly and the optical reception subassembly.
2. Description of the Related Art
A transmission/reception optical module (optical transceiver) is provided with, for example, a circuit board 181 as shown in FIG. 1 wherein the circuit board 181 is formed by a rigid substrate (a substrate which is not flexible, but rigid and not bent) which is formed entirely in the same plane. To one end of the circuit board 181, an LD (semiconductor laser) subassembly 182 and a PD (photodiode) subassembly 183 are fixed. Leads 182R of the LD subassembly 182 and leads 183R of the PD subassembly 183 situated, respectively, at an end of the LD subassembly 182 and an end of the PD subassembly 183 are soldered onto respective subassembly terminals 184 formed on the end of the circuit board 181 to be secured thereto.
The circuit board 181 is incorporated into a casing (not shown). Grooves 185, 185 for positioning screws are provided on opposed sides of the circuit board 181 in a side opposite to that where the LD and PD subassemblies are secured. The circuit board 181 is screwed to the casing so as to fix vertically the circuit board 181 (±z-direction in FIG. 1). A card edge region 186 for attaching the circuit board 181 to communication equipment (not shown) and detaching it from the latter (inserting the circuit board 181 into communication equipment (not shown) and extracting it from the latter) is formed on the other end of the circuit board 181 opposite to that where the LD and PD subassemblies are secured. Connection terminals 187, 187, . . . for connecting electrically the circuit board 181 to the communication equipment are formed in the card edge region 186.
When the card edge region 186 contained in the casing is inserted into the communication equipment along −y direction, the circuit board 181 is electrically connected with the communication equipment. On one hand, when optical connectors each containing an optical fiber functioning as a transmission line (not shown) are connected to each one end of the LD subassembly 182 and the PD subassembly 183, the transmission lines are connected optically to the LD subassembly 182 and the PD subassembly 183, respectively, and they are ready for use.
For the optical connection, each height (vertical position in optic axis, i.e. ±z-direction in FIG. 1) of the LD subassembly 182 and the PD subassembly 183, and a distance D (a distance between the optical axes, i.e. a width (±x) direction in FIG. 1) of the LD subassembly and the PD subassembly 183 are previously determined to have each predetermined dimension.
Information for literary documents of the prior art of the invention according to this application are as follows:
Japanese patent application laid-open Nos. 2001-298217 and 1996-136767
In these circumstances, however, it is difficult to assure the correct distance D which has been previously determined in case of securing the LD subassembly 182 and the PD subassembly 183 to the circuit board 181. Even if both the subassemblies 182 and 183 are secured to the circuit board 181 with the distance D, there is such a problem that a considerable stress s appears on the LD subassembly 182 and the PD subassembly 183 in the case where each height of the LD and PD subassemblies 182 and 183, and the distance D provided between them do not coincide with the previously determined values, respectively. This is because the circuit board 181 is made from a rigid substrate which is formed entirely in the same plane, so that the LD subassembly 182 and the PD subassembly 183 are not in just the right sizes in a hold region 192 of a casing 191 containing the circuit board 181, when the LD and PD subassemblies 182 and 183 are fitted in the hold region 192 to be securely maintained (not perfectly rigid) as shown in FIG. 2A.
Particularly, since the remarkable stress s concentrates in a connection region for the circuit board 181 and the LD and PD subassemblies 182, 183, cracks appear easily in the solder. Accordingly, transmission properties of the LD subassembly 182 and reception properties (e.g. reception sensitivity) of the PD subassembly 183 become inferior. Furthermore, there is a case where optic axis deviation appears on the LD subassembly 182 or the PD subassembly 183 due to the stress s.
Moreover, there is such a case where since a contour of the LD subassembly differs from that of the PD subassembly 183 as shown in FIG. 2B, positions of the leads 182R differ also from those of the leads 183R. These problems as mentioned above cannot be solved fundamentally in the circuit board 181 made entirely from a rigid substrate.
The LD subassembly 182 is composed of an LD device module 301 wherein an LD device being a light-emitting element is contained in a package and a ferrule block (capillary block) 302 which is to be secured to the LD device module 301 so as to align their central axes and to which the above-mentioned optical connector is connected.
However, due to a reason for totalizing precisions of a variety of parts and the like, a central axis of the LD device module 301 deviates from that of the capillary block 302, when the central axes of the LD device module 301 and the capillary block 302 are aligned in, for example, ±x-, z-directions, and the length (±y) direction. Moreover, since a length L of the subassembly 182 is different from that of another subassembly as shown in FIG. 4, there is an inequality in each individual of the LD subassembly 182 wherein the central axis thereof is aligned. In this respect, when the circuit board 181 shown in FIG. 1 is used, deviation in each individual of the LD subassemblies 182 cannot compensate to each other.
When the LD subassembly 182 is secured to the casing 191 (see FIG. 2A), the best situation is in such that the circuit board 181 and the leads 182R secured to the circuit board 181 are in parallel to the horizontal (±x, y) direction of the casing 191.
However, there are a case where the LD subassembly 182 is secured with respect to the horizontal direction of the casing 191 at a deviated angle θ together with the circuit board 181 as shown in FIG. 5B, and a case where only the LD assembly 182 is secured at a deviated angle θ due to positional deviation of the leads 182R. When the circuit board 181 as shown in FIG. 1 is used, a deviation derived from varying results in securing the LD subassembly 182 to the circuit board 181 cannot be responded.
The same problems arise also in the PD subassembly 183 as those described with respect to the LD subassembly 183 by referring to FIGS. 3, 4, 5A, and 5B.
On the other hand, the circuit board 181 shown in FIG. 1 involves such a problem that when a screw accompanies with a backlash in case of attaching the card edge region 186 to or detaching the card edge region 186 from communication equipment (not shown), significant force F1 in +y-direction and force F2 in −y-direction are applied to the circuit board 181. As a result, a stress is applied to a connection section of the LD subassembly 182 and the PD subassembly 183, so that cracks appear easily in a solder.
Meanwhile, a difference in diameters of the LD subassembly 182 and the PD subassembly 183, and a deviation in tolerance of parts can be overcome by bending (forming) properly leads 182R and 183R for the LD subassembly 182 and the PD subassembly 183 in a transmission/reception optical module containing the circuit board 181 shown in FIG. 1 in case of a low speed transmission of a 1 Gbit/s or less in a signal transmission rate.
It is required, however, to make a transmission distance from an end of the circuit board 181 to the LD subassembly 182 or the PD subassembly 183 to be the shortest for achieving a high speed transmission of 5 Gbit/s or more. Particularly, leads to be formed on a side of the LD subassembly cannot be adopted, because transmission properties become inferior.
Although the circuit board 181 may be prepared from a flexible substrate, since such flexible substrate exhibits “arcuation”, so that a transmission distance becomes long, resulting in inferior transmission properties. Accordingly, it is impossible to achieve a high speed transmission at a rate of 5 Gbit/s or higher by adopting simply a flexible substrate.